Engineering Bright and Mechanosensitive Alkaline-Earth Rare-Earth Upconverting Nanoparticles

Abstract

Upconverting nanoparticles (UCNPs) are an emerging platform for mechanical force sensing at the nanometer scale. An outstanding challenge in realizing nanometer-scale mechano-sensitive UCNPs is maintaining a high mechanical force responsivity in conjunction with bright optical emission. This Letter reports mechano-sensing UCNPs based on the lanthanide dopants Yb³⁺ and Er³⁺, which exhibit a strong ratiometric change in emission spectra and bright emission under applied pressure. We synthesize and analyze the pressure response of five different types of nanoparticles, including cubic NaYF₄ host nanoparticles and alkaline-earth host materials CaLuF, SrLuF, SrYbF, and BaLuF, all with lengths of 15 nm or less. By combining optical spectroscopy in a diamond anvil cell with single-particle brightness, we determine the noise equivalent sensitivity (GPa/√Hz) of these particles. The SrYb_0.72Er_0.28F@SrLuF particles exhibit an optimum noise equivalent sensitivity of 0.26 ± 0.04 GPa/√Hz. These particles present the possibility of robust nanometer-scale mechano-sensing.

Figure 1.

(a) Truncated periodic table highlighting Na, Ca, Sr, and Ba, the systematically studied alkaline earth elements used in the host lattice for rare-earth upconverting nanoparticles. (b) Schematic demonstrating the active core and passivating shell architecture of the upconverting nanoparticles. (c) Energy diagram showing near-infrared upconversion using Yb³⁺ and Er³⁺ ions. A Yb³⁺ ion is excited by a 980 nm photon, which can nonradiatively transfer that energy to a nearby Er³⁺ ion. This process can occur multiple times before the Er³⁺ ion decays, allowing the Er³⁺ ion to emit into the visible wavelengths. (d−g) TEM images of NaYF₄, CaLuF, SrLuF, and BaLuF, respectively. The scale bar for the TEM images is 50 nm. Images e−g originally appeared in conference proceedings for the Conference for Lasers and Electro Optics.

Figure 2.

(a−d) Single-particle optical images (left) and brightness histograms (right) of NaYF₄, CaLuF, SrLuF, and BaLuF, respectively. The scale bar for the optical images is 2 μm. Abbreviations are as follows: pps, photons per second; px, pixel.

Figure 3.

(a) Schematic of a diamond anvil cell (DAC) measurement. A 980 nm laser was coupled to a DAC loaded with UCNPs suspended in silicone oil. The upconversion emission was then collected on a spectrometer for color ratiometric measurements or a camera for imaging. Images of cubic NaYF₄ particles in a DAC under (b) 0.0015 and (c) 4.8 GPa and with an excitation laser wavelength of 980 nm. (d) Upconversion spectra normalized to the highest green intensity. All particles demonstrate an increase in the red-to-green ratio upon compression. The lighter lines were measured at 0 GPa, and the darker lines were measured at 4.8, 4.9, 4.8, and 4.9 GPa for NaYF₄, CaLuF, SrLuF, and BaLuF respectively. The spectra for CaLuF, SrLuF, and BaLuF were originally presented in conference proceedings for the Conference for Lasers and Electro Optics. (e) The ratiometric emission (I_Red/I_Green) as a function of the applied pressure. The lines are a linear fit to the data. The fit parameters are found in Table 1. (f) The percent change of the red intensity (I_Red) compared to the green intensity (I_Green) color ratio of the four materials measured across the three pressure cycles. (g) The minimal detectable change in pressure for each material. The labels are simplified to Na, Ca, Sr, and Ba. The scale bars in panels b and c are 50 μm.

Figure 4.

SrYb_0.72Er_0.28F@SrLuF UCNPs for enhanced mechano-sensitivity. (a) TEM micrograph of SrYb_0.72Er_0.28F@SrLuF UCNPs. (b) Wide-field optical image of SrYb_0.72Er_0.28F@SrLuF UCNPs with 980 nm excitation at an irradiance of 27 kW/cm². (c) Histogram of the SrYbF brightness at 27 kW/cm². (d) Optical response of SrYb_0.72Er_0.28F@SrLuF UCNPs under three pressure cycles in a DAC. (e) Mechano-sensitivity at the single-particle level for NaYF4, CaLuF, SrLuF, BaLuF, and SrYb_0.72Er_0.28F@SrLuF (labeled SrYbF) UCNPs based on ensemble DAC measurements and single-particle brightness measurements. The scale bar in image a is 50 nm, and that in image b is 2 μm. Abbreviations are as follows: pps, photons per second; px, pixel.